The present invention relates to a method of monitoring the phase or state of a material in a process including crystallization from a melt and to an apparatus capable of performing such a method.
The present invention can be utilized during zone melting of metals and semiconductors, as well as in growing of single crystals, at pulling of shaped articles from a malt and in a number of other metallurgical and like processes including crystallization treatment of materials.
IT IS KNOWN THAT TO ENSURE A HIGH DEGREE OF PURIFICATION, TO OBTAIN A HIGHLY HOMOGENOUS STRUCTURE AND TO PROVIDE FOR ACCURATE GEOMETRICAL DIMENSIONS OF THE INGOTS PRODUCED, IT IS ESSENTIAL TO DETERMINE WITH A HIGH DEGREE OF ACCURACY THE LOCATION AND MORE OFTEN THAN NOT THE SHAPE OF THE INTERFACE BETWEEN THE SOLID AND LIQUID PHASES. The presence of reliable information on the current coordinates and shape of the interface makes it possible to effect the optimum automatic control of the corresponding production processes.
However, the main difficulty in determining the interface between the liquid and solid phases consists in identification of these phases.
Known in the art are methods of determining the phase states and, hence, the location of the interface between the liquid and solid phases, based on a comparative analysis of either the intensity or of the spectral characteristics of radiation energy coming from the melt and from the solid crystallized structure adjoining this melt.
However, in most hitherto known cases these methods fail to discriminate between the liquid and solid phases with adequate reliability and thus to locate with adequate accuracy the interface therebetween.
Thus, in zone melting of aluminum the factor of the radiant capacity of the crystallized surface of an ingot is a highly unstable value depending on the roughness of the surface, the degree of its oxidation thereof and on other factors which are difficult to monitor. The absolute value of the factor of the blackness of this surface might be the same as the similar characteristic of the surface of molten aluminum, particularly when the latter is covered with oxide and slag films. Consequently, the comparison between the intensities of the flows of radiation energy received by the photo-optical follow-up system, respectively, from the solid and liquid phases fails to determine the position of the interface between the phases.
Another fact which hampers the measurement of the absolute values and spectral characteristics of the flows of radiation energy coming from the surfaces being inspected is the gradual build-up of deposits on the inspection windows through which the process is being followed of dark opaque deposits of volatile impurities, metal vapors, graphite particles and other sublimants. The optical density of these deposits tends to drift in time and to vary over the area of the window, whereby these deposits have introduced both spectrum-wise and amplitude-wise errors which affect the accuracy of the comparative measurements of these flows of radiation energy.
The abovedescribed hitherto known methods are usually performed by apparatus comprising a carriage reciprocable transversely of the interface of the solid/liquid phases, the carriage supporting thereon a photo-optical system. This photo-optical system includes an objective lens and a photoreceiver or phototransducer adapted to receive upon the reciprocation of the carriage the radiation coming either from the solid phase or from the liquid phase of the substance being treated.
Connected to the output of the photoreceiver or phototransducer is an analyzer of the output signal of this photoreceiver or phototransducer. It is supposed that the output signal of the phototransducer would be different, depending on whether it is aimed at the melt or at the solid part of the ingot, and, thus, by detecting the moment of a change of this output signal, it would be possible to monitor the position of the interface between the solid and liquid phases. However, the hitherto known apparatus have the same inherent disadvantages which have been described hereinabove in connection with the methods performed in these apparatus, whereby the character of the variation of the output signal of the phototransducer under the action of various disturbances in a number of instances is not directly related to the position of the photo-optical system with respect to the interface. Therefore, the positions into which the carriage is driven as a result of the analysis of the output signal of the phototransducer are but to a certain extent representative of the actual location of the interface being monitored, the error sometimes attaining significant values.
It is an object of the present invention to create a method providing reliable recognition and identification of the phase states of the material being treated and of the interface between the phases, irrespectively of the change of irradiation properties of the surfaces of these phases.
It is another object of the present invention to preclude the influence of the gradual, time-related darkening of the inspection windows upon the outcome of the measurements.
It is a further object of the present invention to provide an apparatus for practical realization of the herein disclosed method of comparative analysis of the radiation energy flow coming from the solid and liquid phases and for monitoring with a high degree of accuracy the position of the interface between the solid and liquid phases.
It is still another object of the present invention to provide an apparatus for detecting the shape of the interface between the liquid and solid phases.
Among other objects of the invention it is worth to mention that of increasing the efficiency of the process being inspected and of stabilizing the quality of the product of this process.